identify who has sent an email from a Shared Mailbox

I don't think DA can help you here.  You would need to look at the Exchage logs.

http://exchangeserverpro.com/using-exchange-server-mailbox-audit-logs-find-sender-email-shared-mailbox/

Thanks, I am more interested in this from a CA reviewing or DA discovery perspective. If you're trying to search or review for all items sent by X staff member, the results would not include anything sent by X staff member from a shared mailbox? Seems to be a fairly important gap from a discovery perspective.

I see your point, but unfortunately it seems to be a MSFT limitation, would you not agree?  If it is really a concern the company could restrict that ability.